I can answer but don't have a computer right now.

Well i ll be waiting since you seem to be a very knowledgeable person

My guess.. the drill marks in the stock pulley are so that it is balanced on its own... but that doesn't mean that is provides any harmonic balancing to the motor.. once it is installed.

People have been using lightened or underdrive pulleys on Subarus for years and all of them come with the rubber piece in the middle. I had one of the first Vishnu pullies on my 02 WRX for 90K miles and 4+ years with no issues from stock through 350+whp.

I have been around NASIOC for over 10 years and I can't think of a single engine failure anyone chocked up to a lightened pulley.

But I'll add a disclaimer the FA20 is a brand new engine, so of course you take your risks when modifying a vehicle. :)

True. I suppose balancing it might help things along but I'd have to wonder if it has crank-opposing balance as well. Doesn't look like there's enough there to balance against the crank. Most 4cyl cranks I've played with are balanced on their own anyway.

Looking forward to seeing Don's reply too-- I tend to trust his judgement, I bet he's had his mitts in more engines than me.

I doubt Perrin would release a product to the masses without thinking it through. they have been extremely reputable in the subaru community for years, and know their stuff.

but... continue to discuss. more info never hurts

I will add to that. There are also a bunch of other companies that offer this type of LW pulleys and they are basically the same. But like you say, more knowledge is even better.

@PERRIN, @PERRIN_Chris, @PERRIN_ADAM, @PERRIN_Jeff can any of you chime in with your thoughts?

http://blog.perrinperformance.com/my-first-brz-part/

i'm sure most of you guys have read it, but for those that haven't here is a write up from Perrin, it somewhat addresses the dampening issue...

i trust their literature considering its direct from the makers versus "he says, she says"

I know that for Toyota they normally have an internal balance shaft (two) but this eng im not too sure of. Also so you all know Toyota also uses a different type of belt tensioner to Reduce vibrations. this eng has that type of tensioner so i would say it would be safe to use. Toyota use to use harmonic balancers but then stopped.

I agree with the OP. The throttle response is improved, as well as initial acceleration. What's the integral of acceleration? Jerk? Something like that...

Anyway, having little experience with lightweight pullies I was a bit worried about drivability issues, i.e. the revs dropping too quick. I can confirm that there are no issues driving around with this pulley. It just makes the driving experience much better.

well said thank u very much

I had perrin pulley on my sti as well. The response was night and day. However you will notice the car slowing down a lot faster when you let go of the gas.

PERFECT! Im used to that now with my bike and will be verry happy with that

There is always going to be those asking about the vibration dampener. Yes the stock part has one, yes our part doesn’t. We and many others have proven over the years that going to a solid pulley on an EJ (current WRX/STI engine) is perfectly fine. There are no issues at all doing this. On the new FA/FB engine, because of the balanced nature, and less vibration in its design, we feel the same can be expected. While only time will tell, our current testers have had ZERO issues with them.

Off the blog.